Spring Pond Dip
by David Walker, UK.
We've had a pond in our small back garden for about twenty years. It was dug by my brother Ian and although a modest size the wildlife large and small that it supports provides us with constant pleasure. The largest residents are a colony of frogs which breed each year. Local wild birds also enjoy daily washes, especially blackbirds, which seem particularly fastidious about their personal grooming even in the middle of winter.
We regularly take samples to study under the stereo and compound microscope to see what's 'out and about', admittedly in a not particularly rigorous way for serious endeavour but just for pleasure. Populations which come and go during the year include green hydra and stentor. The fauna and flora has built up naturally with the occasional injection of 'post study' pond samples collected further afield.
This is a snapshot of life large and small spotted in samples taken in mid March 2008. My identification skills for many freshwater groups hasn't advanced much beyond family / genus (if lucky) and often satisfied to identify to the closest group by 'matching the picture to the critter'. Any comments on the identifications offered below would be welcomed.
Flagellated algae: Fresh samples from the pond under the compound microscope contained almost a 'monoculture' of an organism I didn't recognise. Contrast enhancement showed two flagella and the organisms were highly motile with rigid bodies. My tentative stab at an ID is Chlorogonium. John et al (ref. 2) illustrates six species in this genus, C. elongatum seems to be closest, which if correct is apparently cosmopolitan but don't recall seeing them before in the pond.
Flagellated algae: Left - 16x objective. Long exposure of 4 secs to give a hint of their dipping out of the plane of focus and forward motion, the streaks are more motile ones. Right - closer view. Lower image - 40x objective, phase. Size (not including flagella) typically 66 x 4.5 µm.
Flagellated protozoan: This protozoan had two distinct flagella of differing lengths, the longer one often trailed (shown below). I'm uncertain of its genus but was fun to watch its antics. It seemed similar to Bodo but this example at 33 µm was twice the size of that typically reported for Bodo 5 - 15 µm (ref. 3).
Flagellated protozoan : 40x objective, phase. Body length 33 µm.
Stalked peritrich: These organisms never fail to appeal with their contractile stalk and 'crown' of cilia creating a vortex in the water. This example was not colonial so perhaps Vorticella although it doesn't look as 'bell-shaped' as have seen previously. The images below show how different contrast enhancement techniques present the organism's structure.
Stalked peritrich: 40x objective. Left - phase, right
- oblique with
offset brightfield aperture of phase condenser.
Stentor: A distinctive organism which was attached to a fragment of decaying grass stalk. Initially I was puzzled by the cilia half way down its body (seen in the oblique image below). But in a few minutes the stentor developed an offspring so may have been the early stages of the youngster's development. (See Walter Dioni's article discussing stentor's reproduction methods.)
Above. Stentor: 16x objective, left - oblique, right - darkfield. The three image sequence below is of the same specimen in phase showing the development of the offspring. The fourth below is another specimen showing green colouration under darkfield.
Left above: '0' mins. Line of cilia seen along edge, midway down body. Right above: 9 mins later. Below: 17 mins later.
The EXIF info embedded into a digicam image is valuable in recalling time of capture, if not recorded at the time.
Copepods: Probably one of the most easily recognised critters seen by eye in a pond sample is a female cyclops with paired egg sacs. Their swimming speed always amazes me. I've never found them easy to photograph live as they often frantically wriggle when placed under a raised coverlslip. A tiny drop of alcohol seems to relax them before returning them to the pond.
cyclops with paired egg sacs. Left - 2.5x objective, compound microscope. Right
- stereo at full zoom of 30x so lacks the 'bite' of a compound
objective. Although a modest stereo scope struggles to show fine detail
of small organisms like copepods and waterfleas particularly to see
critical ID features, it's a good complement to compound microscope
studies to learn about their behaviour in a more normal habitat,
rather than squashed under a coverslip.
Below: Nauplius larvae of a copepod. 10x objective., oblique.
Beetle larva: While studying a pond sample in a Petri dish to seek out smaller critters to isolate for the compound scope, this insect larva (beetle?) was seen skulking in the vegetation. I wouldn't fancy bumping into this critter if I was a microorganism on its lunch menu as it had a fearsome set of mouthparts.
body length including appendages ca. 5 mm.
Left: stereo, digicam held to eyepiece.
Middle: Near infra red >850 nm, visible light blocking filter on tungsten lamp. 2.5x objective.
Right: As above with visible light. Near IR has better transmittance through the exoskeleton, although not as good as I'd hoped cf prepared slides of dense insect mounts I've tried.
Two Coleps recently
divided with only half a lorica each.
Comments to the author David Walker are welcomed.
1. 'Collins Field Guide to Freshwater Life' by R. Fitter and
R. Manuel, 1986.
Now sadly out of print, this book had simple ID guides with thumbnail drawings for members of major groups of microscopic fauna and flora as well as macroscopic, so often possible to identify close to the correct group. If spotted used for about £20 rather than the £100-200 sometimes seen, it's a good buy. Collins recently published a ' Pocket Guide - Freshwater Life' in 2007 but apparently now only covers organisms 'visible with the naked eye'.
2. 'The freshwater algal flora of the British Isles. An identification guide to freshwater and terrestial algae.' Edited by D. M. John, B. A. Whitton and A. J. Brook. Cambridge University Press, 2002.
3. 'A beginner's guide to the collection, isolation, cultivation and identification of freshwater protozoa.' by B. J. Finlay, A. Rogerson and A. J. Cowling. Pub. NERC / FBA 1988.
Microscopy UK Front Page
Published in the March 2008 edition of Micscape.
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